Three-dimensional memory devices having isolation structure for source select gate line and methods for forming the same

ABSTRACT

Embodiments of three-dimensional (3D) memory devices and methods for forming the same are disclosed. In an example, a 3D memory device includes a substrate, a memory stack on the substrate, a plurality of channel structures each extending vertically through the memory stack, an isolation structure, and an alignment mark. The memory stack includes a plurality of interleaved conductive layers and dielectric layers. An outmost one of the conductive layers toward the substrate is a source select gate line (SSG). The isolation structure extends vertically into the substrate and surrounds at least one of the channel structures in a plan view to separate the SSG and the at least one channel structure. The alignment mark extends vertically into the substrate and is coplanar with the isolation structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is continuation of International Application No.PCT/CN2020/113423, filed on Sep. 4, 2020, entitled “THREE-DIMENSIONALMEMORY DEVICES HAVING ISOLATION STRUCTURE FOR SOURCE SELECT GATE LINEAND METHODS FOR FORMING THE SAME,” which is hereby incorporated byreference in its entirety. This application is also related toco-pending U.S. application Ser. No. ______, Attorney Docketing No.:10018-01-0137-US, filed on even date, entitled “THREE-DIMENSIONAL MEMORYDEVICES HAVING ISOLATION STRUCTURE FOR SOURCE SELECT GATE LINE ANDMETHODS FOR FORMING THE SAME,” which is hereby incorporated by referencein its entirety.

BACKGROUND

Embodiments of the present disclosure relate to three-dimensional (3D)memory devices and fabrication methods thereof.

Planar memory cells are scaled to smaller sizes by improving processtechnology, circuit design, programming algorithm, and fabricationprocess. However, as feature sizes of the memory cells approach a lowerlimit, planar process and fabrication techniques become challenging andcostly. As a result, memory density for planar memory cells approachesan upper limit.

A 3D memory architecture can address the density limitation in planarmemory cells. The 3D memory architecture includes a memory array andperipheral devices for controlling signals to and from the memory array.

SUMMARY

Embodiments of 3D memory devices and fabrication methods thereof aredisclosed herein.

In one example, a 3D memory device includes a substrate, a memory stackon the substrate, a plurality of channel structures each extendingvertically through the memory stack, an isolation structure, and analignment mark. The memory stack includes a plurality of interleavedconductive layers and dielectric layers. An outmost one of theconductive layers toward the substrate is a source select gate line(SSG). The isolation structure extends vertically into the substrate andsurrounds at least one of the channel structures in a plan view toseparate the SSG and the at least one channel structure. The alignmentmark extends vertically into the substrate and is coplanar with theisolation structure.

In another example, a 3D memory device includes a substrate, an SSGextending laterally, an isolation structure extending vertically throughthe SSG into the substrate, a first channel structure extendingvertically through the SSG into the substrate, and a second channelstructure extending vertically through the isolation structure into thesubstrate and spaced apart from the SSG by the isolation structure.

In still another example, a method for forming a 3D memory device isdisclosed. An SSG sacrificial layer is formed above a substrate. Anisolation structure and an alignment mark each through the SSGsacrificial layer are simultaneously formed. A plurality of interleavedword line dielectric layers and word line sacrificial layers are formedabove the SSG sacrificial layer, the isolation structure, and thealignment mark. A first channel structure extending vertically throughthe interleaved word line dielectric layers and word line sacrificiallayers and the isolation structure is formed. The word line sacrificiallayers and the SSG sacrificial layer are replaced with a plurality ofconductive layers to form a plurality of word lines and an SSG,respectively, such that the first channel structure is spaced apart fromthe SSG by the isolation structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate embodiments of the present disclosureand, together with the description, further serve to explain theprinciples of the present disclosure and to enable a person skilled inthe pertinent art to make and use the present disclosure.

FIG. 1 illustrates a side view of a cross-section of an intermediatestructure in forming a 3D memory device.

FIGS. 2A-2C illustrate plan view and side view images of cross-sectionsof an intermediate structure in forming a 3D memory device.

FIG. 3 illustrates a side view of a cross-section of an intermediatestructure in forming a 3D memory device having an isolation structurefor SSG, according to some embodiments of the present disclosure.

FIG. 4A illustrates a side view of a cross-section of an exemplary 3Dmemory device having an isolation structure for SSG, according to someembodiments of the present disclosure.

FIG. 4B illustrates a plan view of a cross-section of an exemplary 3Dmemory device having an isolation structure for SSG, according to someembodiments of the present disclosure.

FIGS. 5A-5J illustrate an exemplary fabrication process for forming a 3Dmemory device having an isolation structure for SSG, according to someembodiments of the present disclosure.

FIG. 6 is a flowchart of an exemplary method for forming a 3D memorydevice having an isolation structure for SSG, according to someembodiments.

FIG. 7 is a flowchart of an exemplary method for forming an isolationstructure for SSG in a 3D memory device, according to some embodimentsof the present disclosure.

Embodiments of the present disclosure will be described with referenceto the accompanying drawings.

DETAILED DESCRIPTION

Although specific configurations and arrangements are discussed, itshould be understood that this is done for illustrative purposes only. Aperson skilled in the pertinent art will recognize that otherconfigurations and arrangements can be used without departing from thespirit and scope of the present disclosure. It will be apparent to aperson skilled in the pertinent art that the present disclosure can alsobe employed in a variety of other applications.

It is noted that references in the specification to “one embodiment,”“an embodiment,” “an example embodiment,” “some embodiments,” etc.,indicate that the embodiment described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases do not necessarily refer to the same embodiment. Further,when a particular feature, structure or characteristic is described inconnection with an embodiment, it would be within the knowledge of aperson skilled in the pertinent art to effect such feature, structure orcharacteristic in connection with other embodiments whether or notexplicitly described.

In general, terminology may be understood at least in part from usage incontext. For example, the term “one or more” as used herein, dependingat least in part upon context, may be used to describe any feature,structure, or characteristic in a singular sense or may be used todescribe combinations of features, structures or characteristics in aplural sense. Similarly, terms, such as “a,” “an,” or “the,” again, maybe understood to convey a singular usage or to convey a plural usage,depending at least in part upon context. In addition, the term “basedon” may be understood as not necessarily intended to convey an exclusiveset of factors and may, instead, allow for existence of additionalfactors not necessarily expressly described, again, depending at leastin part on context.

It should be readily understood that the meaning of “on,” “above,” and“over” in the present disclosure should be interpreted in the broadestmanner such that “on” not only means “directly on” something but alsoincludes the meaning of “on” something with an intermediate feature or alayer therebetween, and that “above” or “over” not only means themeaning of “above” or “over” something but can also include the meaningit is “above” or “over” something with no intermediate feature or layertherebetween (i.e., directly on something).

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations), and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

As used herein, the term “substrate” refers to a material onto whichsubsequent material layers are added. The substrate itself can bepatterned. Materials added on top of the substrate can be patterned orcan remain unpatterned. Furthermore, the substrate can include a widearray of semiconductor materials, such as silicon, germanium, galliumarsenide, indium phosphide, etc. Alternatively, the substrate can bemade from an electrically non-conductive material, such as a glass, aplastic, or a sapphire wafer.

As used herein, the term “layer” refers to a material portion includinga region with a thickness. A layer can extend over the entirety of anunderlying or overlying structure or may have an extent less than theextent of an underlying or overlying structure. Further, a layer can bea region of a homogeneous or inhomogeneous continuous structure that hasa thickness less than the thickness of the continuous structure. Forexample, a layer can be located between any pair of horizontal planesbetween, or at, a top surface and a bottom surface of the continuousstructure. A layer can extend laterally, vertically, and/or along atapered surface. A substrate can be a layer, can include one or morelayers therein, and/or can have one or more layer thereupon, thereabove,and/or therebelow. A layer can include multiple layers. For example, aninterconnect layer can include one or more conductor and contact layers(in which interconnect lines and/or via contacts are formed) and one ormore dielectric layers.

As used herein, the term “nominal/nominally” refers to a desired, ortarget, value of a characteristic or parameter for a component or aprocess operation, set during the design phase of a product or aprocess, together with a range of values above and/or below the desiredvalue. The range of values can be due to slight variations inmanufacturing processes or tolerances. As used herein, the term “about”indicates the value of a given quantity that can vary based on aparticular technology node associated with the subject semiconductordevice. Based on the particular technology node, the term “about” canindicate a value of a given quantity that varies within, for example,10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

As used herein, the term “3D memory device” refers to a semiconductordevice with vertically oriented strings of memory cell transistors(referred to herein as “memory strings,” such as NAND memory strings) ona laterally-oriented substrate so that the memory strings extend in thevertical direction with respect to the substrate. As used herein, theterm “vertical/vertically” means nominally perpendicular to the lateralsurface of a substrate.

In some 3D memory devices, in forming channel structures, semiconductorplugs are first formed in the bottom of channel holes using selectiveepitaxial growth (thus also known as SEGs), for example, for channelhole gauging. However, for certain device designs, the quality oruniformity of SEGs in some channel holes may be unsatisfied; SEGs mayeven fail to form in some channel holes. As a result, the SSG and thesubstrate may be short-circuited to cause leakage current (SSG leakage)in the final 3D memory devices, thereby causing device failure.

In an example, during one stage of fabricating a 3D memory device havinga core array region 108, an edge region 110, and a staircase region 112,a dielectric stack 104 is formed on a substrate 102 as shown in FIG. 1.Channel holes 114 are formed extending vertically through dielectricstack 104 into substrate 102 in core array regions 108 and edge region110, but not in staircase region 112 in which the staircase structure isto be formed in the final 3D memory device. Semiconductor plugs 116 areselectively formed in the bottom portion of each channel hole 114 usingepitaxial growth. However, as edge region 110 is laterally between corearray region 108 that is full of channel holes 114 and staircase region112 that is free of channel holes 114, the etch loading effect may causeunsatisfied etching of channel holes 114 in edge region 110 (e.g., withpolymer residuals that are hard to be cleaned up), in particular,channel holes 114 in the one or more outmost columns adjacent tostaircase region 112, which in turn jeopardizes the formation ofsemiconductor plugs 116 in those channel holes 114. As a result, duringthe gate-replacement process, when the bottom-most sacrificial layer 106(also known as the SSG sacrificial layer) of dielectric stack 104 isreplaced by a conductive layer (the SSG), the conductive material canleak into those channel holes 114 that do not have semiconductor plugs116 or have defected semiconductor plugs 116 to short the SSG andsubstrate 102.

For example, in the image of FIG. 2A, semiconductor plugs (in the brightcolor) failed to form in some channel holes (in the dark color) in theoutmost column (in the dashed box) of the core region. Short-circuitbetween the SSG and the substrate thus can occur in those channel holesin the final device. Even if semiconductor plugs can be formed, thequality or uniformity of the semiconductor plugs in those channel holesmay be impaired as well. As shown in the image of FIG. 2B, although asemiconductor plug 202 is formed in the bottom portion of a channel hole204, the quality of semiconductor plug 202 is unsatisfied as it leaves avoid between SSG sacrificial layer 206 and substrate 208, which cancause short-circuit between the SSG and substrate 208 in the finaldevice as well.

Moreover, one or more alignment marks 118 into substrate 102 are formedprior to the formation of dielectric stack 104, which can be used foralignment in later processes, such as when forming the staircasestructure after the formation of the channel structures. However, asshown in the left-side image of FIG. 2C, the trench of an alignment mark210 into substrate 208 may cause a dent (in the dashed oval) at the topsurface of the stack structure (e.g., the lower dielectric deck of adual-deck dielectric stack) right above alignment marks 210. As aresult, in the right-side image of FIG. 2C residuals 212 (e.g., thepolysilicon residual between the lower and upper dielectric decks of thedual-deck dielectric stack) may be trapped into the dent, which blocksthe light to alignment mark 210 underneath for alignment in the laterprocess, for example, in forming the staircase structure. Thus, theproduction yield can be reduced due to alignment failure.

Various embodiments in accordance with the present disclosure provide a3D memory device having an isolation structure for SSG that can avoidSSG leakage. During the fabrication process, by separating the SSGsacrificial layer and the channel hole(s) using one or more isolationstructures in the edge region that is prone to growth failure or defectsof semiconductor plugs before the gate-replacement process, even thegrowth failure or voids in the semiconductor plugs would not turn intoSSG leakage in the final 3D memory device.

FIG. 3 illustrates a side view of a cross-section of an intermediatestructure 300 in forming a 3D memory device having an isolationstructure for SSG, according to some embodiments of the presentdisclosure. As shown in FIG. 3, a dielectric stack 304 including an SSGsacrificial layer 306 (e.g., the bottom-most sacrificial layer) can beformed on a substrate 302. An array of channel holes 312 can be formedeach extending vertically through dielectric stack 304 into substrate302, and a semiconductor plug 314 can be selectively formed in thebottom portion of each channel hole 312. Different from the exampleshown in FIG. 1 in which each channel hole 114, including the one inedge region 110, extends through SSG sacrificial layer 306, such thatSSG sacrificial layer 306 is in contact with channel holes 114 in edgeregion 110, channel hole 312 in the edge region as shown in FIG. 3extends through an isolation structure 316 and thus, is spaced apartfrom SSG sacrificial layer 306 by isolation structure 316 therebetween.As a result, even if semiconductor plug 314 failed to form in channelhole 312 or had voids therein, SSG leakage would not occur due to theexistence of isolation structure 316 laterally between the SSG(replacing SSG sacrificial layer 306) and substrate 302 exposed inchannel hole 312. In some embodiments, isolation structure 316, like ashallow trench isolation (STI), extends through SSG sacrificial layer306 into substrate 302, such that the bottom surface of isolationstructure 316 is below the top surface of substrate 302.

Moreover, the formation of the isolation structures can be in the sameprocess of forming the alignment mark and SSG cut, which does notintroduce additional fabrication process and cost. As a result, theisolation structure, alignment mark, and SSG cut can be coplanar. Insome embodiments, the top surfaces of the isolation structure, alignmentmark, and SSG cut are planarized before the formation of the dielectricstack. The flat surface of the alignment mark thus can prevent the dentat the top surface of the dielectric stack, thereby avoiding theresiduals in the dent that can cause alignment failure in laterprocesses.

In some embodiments, the final 3D memory device after fabrication thusincludes the isolation structure extending vertically into the substrateand surrounding at least one of the channel structures in the edge coreto separate the SSG and the channel structure, as well as the alignmentmark extending vertically into the substrate and coplanar with theisolation structure.

FIG. 4A illustrates a side view of a cross-section of an exemplary 3Dmemory device 400 having an isolation structure for SSG, according tosome embodiments of the present disclosure. 3D memory device 400 may beone example of the final 3D memory device of intermediate structure 300in FIG. 3 after fabrication. FIG. 4B illustrates a plan view of across-section of 3D memory device 400 having an isolation structure forSSG, according to some embodiments of the present disclosure. FIG. 4Bmay illustrate an example of a plan view of the cross-section in the AAplane of 3D memory device 400 in FIG. 4A.

3D memory device 400 can include a substrate 402, which can includesilicon (e.g., single crystalline silicon), silicon germanium (SiGe),gallium arsenide (GaAs), germanium (Ge), silicon on insulator (SOI), orany other suitable materials. In some embodiments, substrate 402 is athinned substrate (e.g., a semiconductor layer), which was thinned froma normal thickness by grinding, wet/dry etching, chemical mechanicalpolishing (CMP), or any combination thereof. It is noted that x-, y-,and z-axes are included in FIG. 4A to illustrate the spatialrelationships of the components in 3D memory device 400. Substrate 402includes two lateral surfaces extending laterally in the x-y plane: afront surface on the front side of the wafer, and a back surface on thebackside opposite to the front side of the wafer. The x- andy-directions are two orthogonal directions in the wafer plane:x-direction is the word line direction, and the y-direction is the bitline direction. The z-axis is perpendicular to both the x- and y-axes.As used herein, whether one component (e.g., a layer or a device) is“on,” “above,” or “below” another component (e.g., a layer or a device)of a semiconductor device (e.g., 3D memory device 400) is determinedrelative to the substrate of the semiconductor device (e.g., substrate402) in the z-direction (the vertical direction perpendicular to the x-yplane) when the substrate is positioned in the lowest plane of thesemiconductor device in the z-direction. The same notion for describingspatial relationships is applied throughout the present disclosure.

3D memory device 400 can include a memory stack 404 on substrate 402.Memory stack 404 can be a stacked storage structure through which NANDmemory strings (e.g., in the form of channel structures 412) are formed.In some embodiments, memory stack 404 includes a plurality ofconductive/dielectric layer pairs stacked vertically on substrate 402.Each conductive/dielectric layer pair can include a conductive layer 403and a dielectric layer 405. That is, memory stack 404 can includeinterleaved conductive layers 403 and dielectric layers 405 stackedvertically. The number of conductive/dielectric layer pairs in memorystack 404 (e.g., 32, 64, 96, 128, 144, 160, 176, 192, 256, etc.) can setthe number of memory cells in 3D memory device 400. It is understoodthat in some embodiments, memory stack 404 may have a multi-deckarchitecture (not shown), which includes a plurality of memory decksstacked over one another. The numbers of the pairs of conductive layers403 and dielectric layers 405 in each memory deck can be the same ordifferent.

Conductive layers 403 can each have the same thickness or have differentthicknesses. Similarly, dielectric layers 405 can each have the samethickness or have different thicknesses. Conductive layers 403 caninclude conductive materials including, but not limited to, tungsten(W), cobalt (Co), copper (Cu), aluminum (Al), polycrystalline silicon(polysilicon), doped silicon, silicides, or any combination thereof.Dielectric layers 405 can include dielectric materials including, butnot limited to, silicon oxide, silicon nitride, silicon oxynitride, orany combination thereof. In some embodiments, conductive layers 403include metals, such as W, and dielectric layers 405 include siliconoxide. It is understood that a silicon oxide film (not shown), such asan in-situ steam generation (ISSG) silicon oxide, is formed betweensubstrate 402 (e.g., a silicon substrate) and memory stack 404,according to some embodiments.

In some embodiments, the outmost ones of conductive layers 403 of memorystack 404 function as the select gate lines to control the source and/ordrain of each NAND memory string (e.g., in the form of channel structure412). The outmost one of conductive layers 403 toward substrate 402(i.e., the bottom-most conductive layer in FIG. 4A) can be an SSG 406(also known as bottom select gate line (BSG)) for control the source ofthe NAND memory string, and the outmost one of conductive layers 403away from substrate 402 (i.e., the top-most conductive layer in FIG. 4A)can be a drain select gate line 407 (DSG, also known as top select gateline (TSG)) for control the drain of the NAND memory string. It isunderstood that the number of conductive layers 403 functioning as theSSG 406 or DSG 407 is not limited to 1 and may be more than 1 in someexamples. It is also understood that although SSG 406 and DSG 407 eachis named as a “line,” the shape is not limited to a one-dimensional linesegment in the x-direction or y-direction and may extend laterally in aplane in two dimensions (e.g., SSG 406 in the x-y plane in FIG. 4B). Theremaining conductive layers 403 of memory stack 404 (e.g., conductivelayers 403 vertically between SSG 406 and DSG 407) are the gatelines/word lines for controlling the memory cells in the NAND memorystring, according to some embodiments.

As shown in FIG. 4A, 3D memory device 400 can include a plurality ofchannel structures 412 each extending vertically through memory stack404 into substrate 402. Channel structure 412 can include a channel holefilled with semiconductor materials (e.g., forming a semiconductorchannel 413) and dielectric materials (e.g., forming a memory film 417).In some embodiments, semiconductor channel 413 includes silicon, such asamorphous silicon, polysilicon, or single crystalline silicon. In someembodiments, memory film 417 is a composite layer including a tunnelinglayer, a storage layer (also known as a “charge trap/storage layer”),and a blocking layer. The remaining space of channel structure 412 canbe partially or fully filled with a capping layer including dielectricmaterials, such as silicon oxide, and/or an air gap. Channel structure412 can have a cylinder shape (e.g., a pillar shape). The capping layer,semiconductor channel 413, the tunneling layer, storage layer, andblocking layer of memory film 417 are arranged radially from the centertoward the outer surface of the pillar in this order, according to someembodiments. The tunneling layer can include silicon oxide, siliconoxynitride, or any combination thereof. The storage layer can includesilicon nitride, silicon oxynitride, silicon, or any combinationthereof. The blocking layer can include silicon oxide, siliconoxynitride, high-k dielectrics, or any combination thereof. In oneexample, memory film 417 can include a composite layer of siliconoxide/silicon oxynitride/silicon oxide (ONO).

In some embodiments, channel structure 412 includes two plugs 414 and415 at a respective end in the vertical direction. As shown in FIG. 4A,channel structure 412 can include a semiconductor plug 414 at one endtoward substrate 402 (i.e., the lower end). As used herein, the “upperend” of a component (e.g., channel structure 412) is the end fartheraway from substrate 402 in the z-direction, and the “lower end” of thecomponent (e.g., channel structure 412) is the end closer to substrate402 in the z-direction when substrate 402 is positioned in the lowestplane of 3D memory device 400. Semiconductor plug 414 can include asemiconductor material, such as single crystalline silicon, that isepitaxially grown from substrate 402. Semiconductor plug 414 canfunction as the controller of the source select gate of the NAND memorystring together with SSG 406. Channel plug 415 can be at the upper endof channel structure 412 and can include semiconductor materials (e.g.,polysilicon). By covering the upper end of channel structure 412 duringthe fabrication of 3D memory device 400, channel plug 415 can functionas an etch stop layer to prevent etching of dielectrics filled inchannel structure 412, such as silicon oxide and silicon nitride. Insome embodiments, channel plug 415 functions as the drain of the NANDmemory string.

As further shown in FIG. 4B, 3D memory device 400 can include core arrayregion 408 and an edge region 410 in the plan view. In some embodiments,3D memory device 400 further includes a staircase region 411, such thatedge region 410 is laterally between staircase region 411 and core arrayregion 408, for example, in the x-direction (the word line direction).That is, edge region 410 can be the transition region between core arrayregion 408 and staircase region 411. Memory stack 404 can include astaircase structure (not shown) in staircase region 411 at which theword lines end for landing word line contacts (not shown). Each “level”of the staircase structure can include one or more conductor/dielectriclayer pairs, each including conductive layer 403 and dielectric layer405. In some embodiments, each two adjacent levels of the staircasestructure are offset by a nominally same distance in the verticaldirection and a nominally same distance in the lateral direction. Foreach two adjacent levels of the staircase structure, the first levelthat is closer to substrate 402 can extend laterally further than thesecond level, thereby forming a “landing area” on the first level forinterconnection in the vertical direction. In some embodiments, channelstructures 412 are disposed in core array region 408 and edge region410, but not in staircase region 411. It is understood that dummychannel structures (not shown) may be formed in staircase region 411 inthe final device of 3D memory device 400 in some examples. But similarto the example in FIG. 1, at a certain stage during the fabricationprocess, staircase region 411 may be free of any channel structure ordummy channel structures, as shown in FIG. 4B.

To mitigate the challenge caused by the etch loading effect and theresulting growth failure or defect issues of semiconductor plug 414 inedge region 410 as described above with respect to FIG. 1, in someembodiments, channel structures 412 have different lateral dimensions(e.g., diameters) in core array region 408 and edge region 410. As shownin FIG. 4B, the lateral dimension (e.g., the diameter or side length) ofedge channel structures 412B (i.e., channel structures 412 disposed inedge region 410) is greater than the lateral dimension (e.g., thediameter or side length) of core channel structures 412A (i.e., channelstructures 412 disposed in core array region 408), according to someembodiments. By increasing the size of the channel holes in edge region410, the polymer residual after etching may be more easily removed bycleaning prior to the formation of semiconductor plugs 414, therebyincreasing the growth quality of semiconductor plugs 414. It isunderstood that the dimensions of edge channel structures 412B and corechannel structures 412A may be the same in some examples. Regardless, insome embodiments, the structures of edge channel structures 412B andcore channel structures 412A are the same, as described above withrespect to channel structure 412. In some embodiments, edge channelstructures 412B are not used as NAND memory strings for storage,although they are similarly electrically connected to other components,like core channel structures 412A, which are used as NAND memory stringsfor storage.

Different from the example in FIG. 1, 3D memory device 400 can includean isolation structure 416 for SSG 406 in edge region 410 to avoid SSGleakage due to the growth failure or defects of semiconductor plugs 414in edge channel structures 412B. As shown in FIG. 4B, isolationstructure 416 surrounds edge channel structures 412B in the plan view toseparate SSG 406 and edge channel structures 412B in edge region 410.Isolation structure 416, however, does not extend to core array region408 or staircase region 411, according to some embodiments. Isolationstructure 416 can include a dielectric including, but not limited to,silicon oxide, silicon nitride, silicon oxynitride, or any combinationthereof. In some embodiments, isolation structure 416 includes the samedielectric material as dielectric layers 405 of memory stack 404, suchas silicon oxide. As shown in FIG. 4B, SSG 406 can extend laterallyacross core array region 408 and edge region 410, and core channelstructure 412A in core array region 408 can be in contact with SSG 406.In contrast, with isolation structure 416 in edge region 410, edgechannel structure 412B in edge region 410 is spaced apart from SSG 406by isolation structure 416, according to some embodiments. It isunderstood that although isolation structure 416 in FIG. 4B surroundsall edge channel structures 412B in edge region 410, it is understoodthat isolation structure 416 in other examples may surround one or someof edge channel structures 412B, such that the remaining edge channelstructures 412B may be still in contact with SSG 406. In someembodiments, as the etch loading effect affects the channel holes in theoutmost column adjacent to staircase region 411 in the plan view themost and gradually decreases in other columns toward core array region408, isolation structure 416 surrounds edge channel structures 412B inthe outmost column adjacent to staircase region 411, but not in othercolumns. It is understood that the number of the outmost columnsurrounded by isolation structure 416 is not limited to 1 and may be anynumber smaller than the total number of columns in edge region 410 insome examples.

As shown in the side view of FIG. 4A, isolation structure 416 extendsvertically through SSG 406 in edge region 410 into substrate 402, suchthat at least one channel structure 412 in edge region 410 is contactwith isolation structure 416, but not SSG 406. In some embodiments,isolation structure 416 is an STI, such that the bottom surface ofisolation structure 416 is below the tops surface of substrate 402. Eachchannel structure 412 in core array region 408 extends verticallythrough DSG 407, the gate lines/word lines of conductive layers 403, andSSG 406, whereas at least one channel structure 412 in edge region 410extends vertically through DSG 407, the gate lines/word lines ofconductive layers 403, and isolation structure 416, instead of SSG 406,according to some embodiments. Because isolation structure 416 extendsfurther into substrate 402, channel structure 412 surrounded byisolation structure 416 in edge region 410 extends into substratefurther than channel structure 412 not surrounded by isolation structure416, such as channel structures 412 in core array region 408, accordingto some embodiments. That is, in some embodiments, the lower end(semiconductor plug 414) of channel structure 412 surrounded byisolation structure 416 in edge region 410 is below the lower end(semiconductor plug 414) of channel structure 412 not surrounded byisolation structure 416, such as channels structures 412 in core arrayregion 408.

In some embodiments, the upper end of semiconductor plugs 414 of eachchannel structure 412 is above SSG 406, such that semiconductor plug 414of channel structure 412 in core array region 408 is in contact with SSG406, and semiconductor plug 414 of at least one channel structure 412 inedge region 410 is in contact with isolation structure 416. The lateraldistance between SSG 406 and at least one channel structure 412 (e.g.,outmost edge channel structures 412B in FIG. 4B) in edge region 410, forexample, the smallest distance between SSG 406 and semiconductor plug414 surrounded by isolation structure 416, is between about 40 nm andabout 80 nm, such as between 40 nm and 80 nm (e.g., 40 nm, 45 nm, 50 nm,55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, any range bounded by the lowerend by any of these values, or in any range defined by any two of thesevalues). That is, channel structures 412 surrounded by isolationstructure 416 in edge region 410 is spaced apart from SSG 406 by atleast 40 nm to 80 nm to avoid SSG leakage, according to someembodiments.

In some embodiments, 3D memory device 400 further includes structuresthat can divide memory stack 404 and channel structures 412 intodifferent areas to be individually controlled by various memoryoperations, such as read, write, program, or erase. As shown in FIG. 4B,3D memory device 400 can include a plurality of parallel slit structures418 (e.g., gate line slits (GLSs)) arranged in the y-direction and eachextending in the x-direction to divide memory stack 404 and channelstructures 412 into multiple memory blocks 401 arranged in they-direction. Although not shown in FIG. 4B, within each memory block401, additional structures, such as DSG cuts or SSG cuts, may furtherdivide memory block 401 into multiple memory fingers. For example, asshown in the side view of FIG. 4A, 3D memory device 400 can furtherinclude SSG cuts 409 extending vertically through SSG 406 into substrate402 to cut off SSG 406. As described below with respect to thefabrication process, as SSG cut 409 and isolation structure 416 can beformed in the same process, SSG cut 409 is coplanar with isolationstructure 416, according to some embodiments. It is understood thatalthough SSG cut 409 is disposed in staircase region 411 in FIG. 4A, SSGcut 409 may be disposed in other regions, such as core array region 408and/or edge region 410, as well. For example, SSG cut 409 may extendlaterally across core array region 408, edge region 410, and staircaseregion 411 to divide SSG 406 into different parts in memory block 401.

In some embodiments, 3D memory device 400 further includes one or morealignment marks 419 each extending vertically through SSG 406 intosubstrate 402. As described below with respect to the fabricationprocess, since alignment mark 419, SSG cut 409, and isolation structure416 can be formed in the same process, alignment mark 419 is coplanarwith SSG cut 409 and isolation structure 416, according to someembodiments. It is understood that alignment marks 419 may be used foralignment during the fabrication of 3D memory device 400, such as theformation of the staircase structure in staircase region 411 after theformation of channel structures 412, and may remain in the final productof 3D memory device 400 despite being unfunctional. It is understoodthat although alignment mark 419 is disposed in staircase region 411 inFIGS. 4A and 4B, alignment mark 419 may be disposed in other regions,such as core array region 408 and/or edge region 410, as well.

It is understood that 3D memory device 400 can include additionalcomponents and structures not shown in FIGS. 4A and 4B including, butnot limited to, local contacts and interconnects in one or moremiddle-end-of-line (MEOL) and back-end-of-line (BEOL) interconnectlayers, and peripheral circuits.

It is understood that the layout design of the isolation structure isnot limited by the example of isolation structure 416 in FIGS. 4A and 4Band may include any other suitable layout designs, such as square,rectangle, circle, cross-shaped, donut-shaped, ring-shaped, etc. It isalso understood that the number of the isolation structure is notlimited by the example of single isolation structure 416 in FIGS. 4A and4B and may be multiple isolation structures. For example, each of one ormore of the channel structures in the edge region may be surrounded byan individual isolation structure, as opposed to sharing one isolationstructure.

FIGS. 5A-5J illustrate an exemplary fabrication process for forming a 3Dmemory device having an isolation structure for SSG, according to someembodiments of the present disclosure. FIG. 6 is a flowchart of anexemplary method for forming a 3D memory device having an isolationstructure for SSG, according to some embodiments. Examples of the 3Dmemory device depicted in FIGS. 5A-5J and 6 include 3D memory device 400depicted in FIGS. 4A and 4B. FIGS. 5A-5J and 6 will be describedtogether. It is understood that the operations shown in method 600 arenot exhaustive and that other operations can be performed as wellbefore, after, or between any of the illustrated operations. Further,some of the operations may be performed simultaneously, or in adifferent order than shown in FIG. 6.

In some embodiments, a dielectric stack including a plurality ofinterleaved dielectric layers and sacrificial layers is formed on asubstrate. The bottom-most one of the sacrificial layers is penetratedthrough by an isolation structure, according to some embodiments. Asshown in FIG. 3, dielectric stack 304 is formed on substrate 302, andSSG sacrificial layer 306, the bottom-most sacrificial layers, ispenetrated through by isolation structure 316. The formation of thedielectric stack can include the formation of the isolation structurethrough the SSG sacrificial layer into the substrate, and the formationof interleaved word line dielectric layers and word line sacrificiallayers above the SSG sacrificial layer and the isolation structure, asdescribed below in detail.

Referring to FIG. 6, method 600 starts at operation 602, in which an SSGsacrificial layer is formed above a substrate. Method 600 proceeds tooperation 604, as illustrated in FIG. 6, in which an isolation structureand an alignment mark each through the SSG sacrificial layer aresimultaneously formed. In some embodiments, an SSG cut through the SSGsacrificial layer is formed in the same process of forming the isolationstructure and the alignment mark. FIG. 7 is a flowchart of an exemplarymethod for forming an isolation structure for SSG in a 3D memory device,according to some embodiments of the present disclosure. Tosimultaneously form the isolation structure and the alignment mark eachthrough the SSG sacrificial layer, at operation 702, the SSG sacrificiallayer, a buffer layer, and a stop layer are sequentially formed abovethe substrate. In some embodiments, an SSG dielectric layer is formed onthe substrate prior to the formation of the SSG sacrificial layer. Insome embodiments, a cap layer is formed on the stop layer after theformation of the stop layer. In some embodiments, the SSG sacrificiallayer and stop layer include silicon nitride, and the SSG dielectriclayer, buffer layer, and cap layer include silicon oxide. The substratecan be a silicon substrate.

As illustrated in FIG. 5A, an SSG dielectric layer 504, an SSGsacrificial layer 506, a buffer layer 508, a stop layer 510, a cap layer512, and a hard mask 514 are sequentially formed on a silicon substrate502. In some embodiments, layers of silicon oxide and silicon nitrideare alternatingly deposited on silicon substrate 502 using one or morethin film deposition processes including, but not limited to, physicalvapor deposition (PVD), chemical vapor deposition (CVD), atomic layerdeposition (ALD), or any combination thereof, such that SSG sacrificiallayer 506 and stop layer 510 include silicon nitride, and SSG dielectriclayer 504, buffer layer 508, and cap layer 512 include silicon oxide. Insome embodiments, hard mask 514 includes one or more layers, such asspin-on-carbon (SOC) and silicon oxynitride, as a planarization layerand an anti-reflection layer, which can be formed using spin coating orany other thin film deposition processes including, but not limited to,PVD, CVD, ALD, or any combination thereof. Hard mask 514 can be used asthe etch mask in the later process as well.

Referring back to FIG. 7, at operation 704, an isolation trench and analignment trench each through the stop layer, the buffer layer, and theSSG sacrificial layer into the substrate are simultaneously etched. Insome embodiments, an SSG cut trench is etched through the stop layer,the buffer layer, and the SSG sacrificial layer into the substrate aswell in the same process. As illustrated in FIG. 5B, an isolation trench516, an SSG cut trench 518, and an alignment trench 519 are etchedthrough cap layer 512, stop layer 510, buffer layer 508, SSG sacrificiallayer 506, and SSG dielectric layer 504 into silicon substrate 502. Itis understood that isolation trench 516 may be patterned to any suitableisolation structure layout designs, such as the examples disclosedherein, using lithography. Hard mask 514 can be patterned according tothe layout design of isolation trench 516, SSG cut trench 518, andalignment trench 519. In some embodiments, the exposed parts of caplayer 512, stop layer 510, buffer layer 508, SSG sacrificial layer 506,and SSG dielectric layer 504 underneath patterned hard mask 514 are thenetched using dry etching and/or wet etching process, such as reactiveion etch (RIE) until reaching silicon substrate 502. To form analignment mark, the top portions of silicon substrate 502 are etched aswell, such that alignment trench 519, SSG cut trench 518, and isolationtrench 516 extend further into silicon substrate 502, according to someembodiments. As formed using the same process, alignment trench 519, SSGcut trench 518, and isolation trench 516 can be coplanar with oneanother.

Referring back to FIG. 7, at operation 706, a dielectric layer isdeposited to fill the isolation trench and the alignment trench. In someembodiments, the SSG cut trench is filled with the dielectric layer aswell in the same process. As illustrated in FIG. 5C, a dielectric layer520, such as a silicon oxide layer, the same material as SSG dielectriclayer 504, buffer layer 508, and cap layer 512, is deposited to fillisolation trench 516, SSG cut trench 518, and alignment trench 519(shown in FIG. 5B) using one or more thin film deposition processesincluding, but not limited to, PVD, CVD, ALD, or any combinationthereof. The thickness of dielectric layer 520 is large enough to ensurethat isolation trench 516, SSG cut trench 518, and alignment trench 519are fully filled, according to some embodiments.

Referring back to FIG. 7, at operation 708, the dielectric layer isplanarized, stopping at the stop layer. As illustrated in FIG. 5D,dielectric layer 520 is planarized until stop layer 510 using wetetching and/or CMP. In some embodiments, since both dielectric layer 520and cap layer 512 have the same material, such as silicon oxide, whichis different from stop layer 510, such as silicon nitride, stop layer510 acts as the CMP stop layer and/or etch stop layer to control thestop of the planarization process to avoid damaging SSG sacrificiallayer 506 underneath.

Referring back to FIG. 7, at operation 710, the stop layer is removed.As illustrated in FIG. 5E, stop layer 510 (shown in FIG. 5D) is removedusing wet etching. In some embodiments in which stop layer 510 includessilicon nitride and dielectric layer 520 and buffer layer 508 includesilicon oxide, phosphoric acid is used as the etchant to selectivelyetch stop layer 510, leaving dielectric layer 520 and buffer layer 508intact.

Referring back to FIG. 7, at operation 712, the dielectric layer and thebuffer layer are planarized to form the isolation structure and thealignment mark. In some embodiments, the SSG cut is formed as well inthe same process. As illustrated in FIG. 5F, another planarizationprocess is applied to planarize dielectric layer 520 (shown in FIG. 5E)and buffer layer 508 using wet etching and/or CMP to form an isolationstructure 522, an SSG cut 524, and an alignment mark 525 each throughSSG sacrificial layer 506 into silicon substrate 502. In someembodiments, SSG sacrificial layer 506 is cut off by SSG cut 524. It isunderstood that buffer layer 508 may be completely removed by theplanarization process or partially left on SSG sacrificial layer 506 indifferent examples. As a result, isolation structure 522, SSG cut 524,and alignment mark 525 can be coplanar with one another. Theplanarization process can also ensure a flat surface of isolationstructure 522, SSG cut 524, and alignment mark 525 to avoid any dent inthe upper structure in later processes caused by, for example, alignmenttrench 519.

Method 600 proceeds to operation 606, as illustrated in FIG. 6 in whicha plurality of interleaved word line dielectric layers and word linesacrificial layers are formed above the SSG sacrificial layer, theisolation structure, and the alignment mark. A dielectric stackincluding interleaved word line dielectric layers and word linesacrificial layers as well as the SSG sacrificial layer can be therebyformed above the substrate.

As illustrated in FIG. 5G, a dielectric stack 526 including a pluralityof dielectric/sacrificial layer pairs is formed on silicon substrate502. In some embodiments, word line sacrificial layers 527 and word linedielectric layers 529 are alternatingly deposited above SSG sacrificiallayer 506, isolation structure 522, and alignment mark 525 (and SSG cut524 in some examples) using one or more thin film deposition processesincluding, but not limited to, PVD, CVD, ALD, or any combinationthereof. In some embodiments, word line sacrificial layers 527 includesilicon nitride (the same material as SSG sacrificial layer 506), andword line dielectric layers 529 include silicon oxide. It is understoodthat the sequence of depositing word line sacrificial layers 527 andword line dielectric layers 529 is not limited. The deposition can startwith word line sacrificial layers 527 or word line dielectric layers529, for example, depending on whether SSG sacrificial layer 506 iscovered with buffer layer 508 (shown in FIG. 5F). As a result,dielectric stack 526 including interleaved word line dielectric layers529 and word line sacrificial layers 527 as well as SSG sacrificiallayer 506 is thereby formed above silicon substrate 502, according tosome embodiments. The bottom-most one of the sacrificial layers ofdielectric stack 526, i.e., SSG sacrificial layer 506, is penetratedthrough by isolation structure 522 and alignment mark 525 and is cut offby SSG cut 524, according to some embodiments. Due to the flat surfaceof alignment mark 525, dents and residuals trapped therein right abovealignment mark 525 can be avoided on the top surface of dielectric stack526.

In some embodiments, after the formation of the dielectric stack, aplurality of channel structures each extending vertically through thedielectric stack into the substrate are formed, such that at least oneof the channel structures extends through the isolation structure. Theformation of the channel structures can include the formations ofchannel holes and semiconductor plugs, as described below in detail.

Method 600 proceeds to operation 608, as illustrated in FIG. 6, in which(i) a first channel structure extending vertically through theinterleaved word line dielectric layers and word line sacrificial layersand the SSG sacrificial layer, and (ii) a second channel structureextending vertically through the interleaved word line dielectric layersand word line sacrificial layers and the isolation structure, aresimultaneously formed. The lateral dimension of the second channelstructure can be greater than the lateral dimension of the first channelstructure. In some embodiments, to simultaneously form the first andsecond channel structures, (i) a first channel hole extending verticallythrough the interleaved word line dielectric layers and word linesacrificial layers and the SSG sacrificial layer into the substrate, and(ii) a second channel hole extending vertically through the interleavedword line dielectric layers and word line sacrificial layers and theisolation structure into the substrate, are simultaneously formed, and(i) a first semiconductor plug in a bottom portion of the first channelhole in contact with the SSG sacrificial layer, and (ii) a secondsemiconductor plug in a bottom portion of the second channel hole spacedapart from the SSG sacrificial layer, are simultaneously formed. In someembodiments, to simultaneously form the first and second semiconductorplugs, the first and second semiconductor plugs are epitaxially grownfrom the substrate in the first and second channel holes, respectively.In some embodiments, the second semiconductor plug is spaced apart fromthe SSG sacrificial layer by the isolation structure.

As illustrated in FIG. 5H, an array of channel holes 528 are formed,each of which extends vertically through interleaved word linedielectric layers 529 and word line sacrificial layers 527 of dielectricstack 526. Channel holes 528 in core array region 531 can extend furtherthrough SSG sacrificial layer 506 into silicon substrate 502. Channelhole 528 in edge region 533 aligned with isolation structure 522 canextend further through isolation structure 522, instead of SSGsacrificial layer 506, into silicon substrate 502. In some embodiments,channel hole 528 extending through isolation structure 522 extends intosilicon substrate 502 further than channel hole 528 extending throughSSG sacrificial layer 506. That is, the lower end of channel hole 528extending through isolation structure 522 can be lower than that ofchannel hole 528 extending through SSG sacrificial layer 506. Channelholes 528 can be patterned using lithography and etched through theinterleaved layers of silicon oxide and silicon nitride using dryetching and/or wet etching processes, such as deep RIE (DRIE). In someembodiments, channel hole 528 extends further through the top portion ofsilicon substrate 502. It is understood that the etching process throughdielectric stack 526 may continue to etch part of silicon substrate 502.In some embodiments, a separate etching process is used to etch part ofsilicon substrate 502 after etching through dielectric stack 526. Insome embodiments, the lateral dimension (e.g., the diameter) of channelhole 528 in edge region 533 is greater than that of channel hole 528 incore array region 531 to make the clean of etching residuals in channelhole 528 in edge region 533 easier as more etching residuals may beformed in channel hole 528 in edge region 533 than in core array region531 due to the etch loading effect.

As illustrated in FIG. 5H, a semiconductor plug 530 is formed in thebottom portion of each channel hole 528 by filling the lower portion ofchannel hole 528 with a semiconductor material (e.g., single crystallinesilicon epitaxially grown from silicon substrate 502) in any suitabledirections (e.g., from the bottom surface and/or side surface). Thefabrication processes for epitaxially growing semiconductor plug 530 caninclude, but not limited to, vapor-phase epitaxy (VPE), liquid-phaseepitaxy (LPE), molecular-beam epitaxy (MBE), or any combinationsthereof, following a cleaning process of removing the etching residualsin channel hole 528. Compared with the example in FIG. 1, by formingisolation structure 522 through SSG sacrificial layer 506 in edge region533, even if semiconductor plug 530 failed to grow or had voids inchannel hole 528 in edge region 533, SSG sacrificial layer 506 would bespaced apart from channel hole 528 to avoid SSG leakage in the laterprocess, thereby increasing the production yield.

As illustrated in FIG. 5I, channel structures 538 are formed in channelholes 528 (shown in FIG. 5H). A memory film 532 (including a blockinglayer, a storage layer, and a tunneling layer) and a semiconductorchannel 534 are sequentially formed in this order along sidewalls ofeach channel hole 528 and on respective semiconductor plug 530. In someembodiments, memory film 532 is first deposited along the sidewalls ofchannel hole 528 and on semiconductor plug 530, and semiconductorchannel 534 is then deposited over memory film 532. The blocking layer,storage layer, and tunneling layer can be sequentially deposited in thisorder using one or more thin film deposition processes, such as ALD,CVD, PVD, any other suitable processes, or any combination thereof, toform memory film 532. Semiconductor channel 534 can then be formed bydepositing a semiconductor material, such as polysilicon, over thetunneling layer of memory film 532 using one or more thin filmdeposition processes, such as ALD, CVD, PVD, any other suitableprocesses, or any combination thereof. In some embodiments, a firstsilicon oxide layer, a silicon nitride layer, a second silicon oxidelayer, and a polysilicon layer (a “SONO” structure) are sequentiallydeposited to form memory film 532 and semiconductor channel 534.

As illustrated in FIG. 5I, a capping layer is formed in channel hole 528(shown in FIG. 5H) and over semiconductor channel 534 to completely orpartially fill channel hole 528 (e.g., without or with an air gap). Thecapping layer can be formed by depositing a dielectric material, such assilicon oxide, using one or more thin film deposition processes, such asALD, CVD, PVD, any other suitable processes, or any combination thereof.A channel plug 536 can then be formed in the top portion of channel hole528. A recess then can be formed in the top portion of channel hole 528by wet etching and/or drying etching parts of memory film 532,semiconductor channel 534, and the capping layer in the top portion ofchannel hole 528. Channel plug 536 can then be formed by depositingsemiconductor materials, such as polysilicon, into the recess by one ormore thin film deposition processes, such as CVD, PVD, ALD, or anycombination thereof. Channel structures 538 each including semiconductorplug 530, memory film 532, semiconductor channel 534, and channel plug536 are thereby formed, according to some embodiments. In someembodiments, channel structure 538 in core array region 531 extendsvertically through dielectric stack 526 with SSG sacrificial layer 506into silicon substrate 502, whereas channel structure 538 in edge region533 extends vertically through isolation structure 522 and dielectricstack 526 without SSG sacrificial layer 506 into silicon substrate 502.

In some embodiments, a staircase structure (not shown) can be formed,for example, on the edge of dielectric stack 526. The staircasestructure can be formed by performing a plurality of so-called“trim-etch” cycles to the dielectric layer pairs of dielectric stack 526toward silicon substrate 502. Due to the repeated trim-etch cyclesapplied to the dielectric layer pairs of dielectric stack 526,dielectric stack 526 can have one or more tilted edges and a topdielectric layer pair shorter than the bottom one. In some embodiments,alignment mark 525 is used for alignment during the formation of thestaircase structure. Due to the flat surface of alignment mark 525,dents and residuals trapped therein can be avoided on the top surface ofdielectric stack 526 to avoid the blockage of alignment mark 525.

In some embodiments, the dielectric stack is replaced with a memorystack by replacing the sacrificial layers with a plurality of conductivelayers, respectively, such that the at least one channel structure isspaced apart from a bottom-most one of the conductive layers by theisolation structure, as described below in detail.

Method 600 proceeds to operation 610, as illustrated in FIG. 6, in whichthe word line sacrificial layers and the SSG sacrificial layer arereplaced with a plurality of conductive layers to form a plurality ofword lines and an SSG, respectively, such that the first channelstructure is in contact with the SSG, and the second channel structureis spaced apart from the SSG by the isolation structure.

As illustrated in FIG. 5J, dielectric stack 526 (shown in FIG. 5I) isreplaced with a memory stack 540 including interleaved word lines 542and word line dielectric layers 529 and an SSG 544 by replacing wordline sacrificial layers 527 (shown in FIG. 5H) with word lines 542 andreplacing SSG sacrificial layer 506 (shown in FIG. 5I) with SSG 544. Insome embodiments, an opening (slit) can be etched through dielectricstack 526 using wet etching and/or dry etching of dielectrics (e.g.,silicon oxide and silicon nitride), such as DRIE. The opening can beused as the pathway for the so-called gate replacement process thatreplaces the sacrificial layers (e.g., word line sacrificial layers 527and SSG sacrificial layer 506) with conductive layers (e.g., word lines542 and SSG 544). The replacement of the sacrificial layers with theconductive layers can be performed by wet etching the sacrificial layers(e.g., silicon nitride) selective to dielectric layers 529 (e.g.,silicon oxide) and filling the etching-resulted recesses with theconductive layers (e.g., W). The conductive layers can be deposited byPVD, CVD, ALD, electrochemical depositions, or any combination thereof.

As a result of replacing SSG sacrificial layer 506 with SSG 544 (i.e.,the bottom-most conductive layer of memory stack 540), channel structure538 in core array region 531 thereby extends through and in contact withSSG 544, according to some embodiments. In contrast, channel structure538 in edge region 533 is still spaced apart from SSG 544 by isolationstructure 522. During the gate replacement process, because channelstructure 538 in edge region 533 is spaced apart from SSG sacrificiallayer 506 by isolation structure 522, even if semiconductor plug 530failed to grow or had voids in channel hole 528, the conductive materialforming SSG 544 would not leak from the recess after removing SSGsacrificial layer 506 to silicon substrate 502 to cause short-circuit.

According to one aspect of the present disclosure, a 3D memory deviceincludes a substrate, a memory stack on the substrate, a plurality ofchannel structures each extending vertically through the memory stack,an isolation structure, and an alignment mark. The memory stack includesa plurality of interleaved conductive layers and dielectric layers. Anoutmost one of the conductive layers toward the substrate is a sourceselect gate line (SSG). The isolation structure extends vertically intothe substrate and surrounds at least one of the channel structures in aplan view to separate the SSG and the at least one channel structure.The alignment mark extends vertically into the substrate and is coplanarwith the isolation structure.

In some embodiments, the plurality of channel structures are disposed ina core array region and an edge region in a plan view, and the at leastone channel structure is disposed in the edge region.

In some embodiments, the memory stack includes a staircase structure,the edge region is laterally between the staircase structure and thecore array region, and the at least one channel structure is disposed inan outmost column adjacent to the staircase structure in the plan view.

In some embodiments, a lateral dimension of the at least one channelstructure is greater than a lateral dimension of the channel structuresdisposed in the core array region.

In some embodiments, a lateral distance between the SSG and the at leastone channel structure is between about 40 nm and about 80 nm.

In some embodiments, each of the channel structures includes asemiconductor plug at one end toward the substrate.

In some embodiments, the isolation structure is laterally between theSSG and the semiconductor plug of the at least one channel structure.

In some embodiments, the semiconductor plug of the at least one channelstructure extends into the substrate further than a semiconductor plugof another one of the channel structures.

In some embodiments, the isolation structure and the alignment mark eachinclude a dielectric.

In some embodiments, the alignment mark extends vertically through theSSG.

In some embodiments, the 3D memory device further includes an SSG cutextending vertically into the substrate and coplanar with the isolationstructure and the alignment mark.

According to another aspect of the present disclosure, a 3D memorydevice includes a substrate, an SSG extending laterally, an isolationstructure extending vertically through the SSG into the substrate, afirst channel structure extending vertically through the SSG into thesubstrate, and a second channel structure extending vertically throughthe isolation structure into the substrate and spaced apart from the SSGby the isolation structure.

In some embodiments, the first channel structure is disposed in a corearray region, and the second channel structure is disposed in an edgeregion in a plan view.

In some embodiments, the 3D memory device further includes a memorystack including a plurality of interleaved conductive layers anddielectric layers. The SSG is an outermost one of the conductive layerstoward the substrate, according to some embodiments.

In some embodiments, the memory stack includes a staircase structure,the edge region is laterally between the staircase structure and thecore array region, and the second channel structure in the edge regionis disposed in an outmost column adjacent to the staircase structure inthe plan view.

In some embodiments, each of the first and second channel structuresincludes a semiconductor plug at one end thereof, the semiconductor plugof the first channel structure is in contact with the SSG, and thesemiconductor plug of the second channel structure is in contact withthe isolation structure.

In some embodiments, the second channel structure extends into thesubstrate further than the first channel structures.

In some embodiments, the 3D memory device further includes an alignmentmark extending vertically through the SSG into the substrate andcoplanar with the isolation structure.

In some embodiments, the isolation structure and the alignment mark eachinclude a dielectric.

In some embodiments, the 3D memory device further includes an SSG cutextending vertically through the SSG into the substrate and coplanarwith the isolation structure and the alignment mark.

In some embodiments, a lateral dimension of the second channelstructures is greater than a lateral dimension of the first channelstructure.

In some embodiments, a lateral distance between the SSG and the secondchannel structure is between about 40 nm and about 80 nm.

According to still another aspect of the present disclosure, a methodfor forming a 3D memory device is disclosed. An SSG sacrificial layer isformed above a substrate. An isolation structure and an alignment markeach through the SSG sacrificial layer are simultaneously formed. Aplurality of interleaved word line dielectric layers and word linesacrificial layers are formed above the SSG sacrificial layer, theisolation structure, and the alignment mark. A first channel structureextending vertically through the interleaved word line dielectric layersand word line sacrificial layers and the isolation structure is formed.The word line sacrificial layers and the SSG sacrificial layer arereplaced with a plurality of conductive layers to form a plurality ofword lines and an SSG, respectively, such that the first channelstructure is spaced apart from the SSG by the isolation structure.

In some embodiments, a second channel structure extending verticallythrough the interleaved word line dielectric layers and word linesacrificial layers and the SSG sacrificial layer is formed in a sameprocess for forming the first channel structure. In some embodiments,the second channel structure is in contact with the SSG by replacing theword line sacrificial layers and the SSG sacrificial layer with theplurality of conductive layers to form the plurality of word lines andthe SSG.

In some embodiments, a buffer layer and a stop layer are sequentiallyformed on the SSG sacrificial layer.

In some embodiments, to form the isolation structure, an isolationtrench and an alignment mark each through the stop layer, the bufferlayer, and the SSG sacrificial layer into the substrate aresimultaneously formed, a dielectric layer is deposited to fill theisolation trench and the alignment trench, the dielectric layer isplanarized stopping at the stop layer, and the stop layer is removed.

In some embodiments, after removing the stop layer, the dielectric layerand the buffer layer are planarized to form the isolation structure andthe alignment mark.

In some embodiments, an SSG cut through the SSG sacrificial layer isformed in a same process of forming the isolation structure and thealignment mark.

In some embodiments, the SSG sacrificial layer includes silicon nitride,and the isolation structure and the alignment mark include siliconoxide.

In some embodiments, to form the first and second channel structures inthe same process, a first channel hole extending vertically through theinterleaved word line dielectric layers and word line sacrificial layersand the isolation structure into the substrate, and a second channelhole extending vertically through the interleaved word line dielectriclayers and word line sacrificial layers and the SSG sacrificial layerinto the substrate, are simultaneously formed, and a first semiconductorplug in a bottom portion of the first channel hole spaced apart from theSSG sacrificial layer, and a second semiconductor plug in a bottomportion of the second channel hole in contact with the SSG sacrificiallayer, are simultaneously formed.

In some embodiments, to simultaneously form the first and secondsemiconductor plugs, the first and second semiconductor plugs areepitaxially grown from the substrate in the first and second channelholes, respectively.

In some embodiments, the first channel hole extends into the substratefurther than the second channel hole.

In some embodiments, a lateral dimension of the first channel structureis greater than a lateral dimension of the second channel structure.

The foregoing description of the specific embodiments will so reveal thegeneral nature of the present disclosure that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent disclosure. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

Embodiments of the present disclosure have been described above with theaid of functional building blocks illustrating the implementation ofspecified functions and relationships thereof. The boundaries of thesefunctional building blocks have been arbitrarily defined herein for theconvenience of the description. Alternate boundaries can be defined solong as the specified functions and relationships thereof areappropriately performed.

The Summary and Abstract sections may set forth one or more but not allexemplary embodiments of the present disclosure as contemplated by theinventor(s), and thus, are not intended to limit the present disclosureand the appended claims in any way.

The breadth and scope of the present disclosure should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. A three-dimensional (3D) memory device,comprising: a substrate; a memory stack on the substrate comprising aplurality of interleaved conductive layers and dielectric layers, anoutermost one of the conductive layers toward the substrate being asource select gate line (SSG); a plurality of channel structures eachextending vertically through the memory stack; an isolation structureextending vertically into the substrate and surrounding at least one ofthe channel structures in a plan view to separate the SSG and the atleast one channel structure; and an alignment mark extending verticallyinto the substrate and coplanar with the isolation structure.
 2. The 3Dmemory device of claim 1, wherein the plurality of channel structuresare disposed in a core array region and an edge region in a plan view,and the at least one channel structure is disposed in the edge region.3. The 3D memory device of claim 2, wherein the memory stack comprises astaircase structure, the edge region is laterally between the staircasestructure and the core array region, and the at least one channelstructure is disposed in an outmost column adjacent to the staircasestructure in the plan view.
 4. The 3D memory device of claim 2, whereina lateral dimension of the at least one channel structure is greaterthan a lateral dimension of the channel structures disposed in the corearray region.
 5. The 3D memory device of claim 1, wherein a lateraldistance between the SSG and the at least one channel structure isbetween about 40 nm and about 80 nm.
 6. The 3D memory device of claim 1,wherein each of the channel structures comprises a semiconductor plug atone end toward the substrate.
 7. The 3D memory device of claim 6,wherein the isolation structure is laterally between the SSG and thesemiconductor plug of the at least one channel structure.
 8. The 3Dmemory device of claim 6, wherein the semiconductor plug of the at leastone channel structure extends into the substrate further than asemiconductor plug of another one of the channel structures.
 9. The 3Dmemory device of claim 1, wherein the isolation structure and thealignment mark each comprise a dielectric.
 10. The 3D memory device ofclaim 1, wherein the alignment mark extends vertically through the SSG.11. The 3D memory device of claim 1, further comprising an SSG cutextending vertically into the substrate and coplanar with the isolationstructure and the alignment mark.
 12. A three-dimensional (3D) memorydevice, comprising: a substrate; a source select gate line (SSG)extending laterally; an isolation structure extending vertically throughthe SSG into the substrate; a first channel structure extendingvertically through the SSG into the substrate; and a second channelstructure extending vertically through the isolation structure into thesubstrate and spaced apart from the SSG by the isolation structure. 13.The 3D memory device of claim 12, wherein the first channel structure isdisposed in a core array region, and the second channel structure isdisposed in an edge region in a plan view.
 14. The 3D memory device ofclaim 12, wherein the second channel structure extends into thesubstrate further than the first channel structures.
 15. The 3D memorydevice of claim 12, further comprising: an alignment mark extendingvertically through the SSG into the substrate and coplanar with theisolation structure; and an SSG cut extending vertically through the SSGinto the substrate and coplanar with the isolation structure and thealignment mark.
 16. A method for forming a three-dimensional (3D) memorydevice, comprising: forming a source select gate line (SSG) sacrificiallayer above a substrate; simultaneously forming an isolation structureand an alignment mark each through the SSG sacrificial layer into thesubstrate; forming a plurality of interleaved word line dielectriclayers and word line sacrificial layers above the SSG sacrificial layer,the isolation structure, and the alignment mark; forming a first channelstructure extending vertically through the interleaved word linedielectric layers and word line sacrificial layers and the isolationstructure; and replacing the word line sacrificial layers and the SSGsacrificial layer with a plurality of conductive layers to form aplurality of word lines and an SSG, respectively, such that the firstchannel structure is spaced apart from the SSG by the isolationstructure.
 17. The method of claim 16, further comprising forming asecond channel structure extending vertically through the interleavedword line dielectric layers and word line sacrificial layers and the SSGsacrificial layer in a same process for forming the first channelstructure, wherein the second channel structure is in contact with theSSG by replacing the word line sacrificial layers and the SSGsacrificial layer with the plurality of conductive layers to form theplurality of word lines and the SSG.
 18. The method of claim 17, whereinsimultaneously forming the isolation structure and the alignment markcomprises: simultaneously etching an isolation trench and an alignmenttrench each through the stop layer, the buffer layer, and the SSGsacrificial layer into the substrate; depositing a dielectric layer tofill the isolation trench and the alignment trench; planarizing thedielectric layer stopping at the stop layer; and removing the stoplayer.
 19. The method of claim 18, further comprising, after removingthe stop layer, planarizing the dielectric layer and the buffer layer toform the isolation structure and the alignment mark.
 20. The method ofclaim 16, further comprising forming an SSG cut through the SSGsacrificial layer in a same process of forming the isolation structureand the alignment mark.